1. Field of the Invention
The present invention is directed to a series radio frequency (RF) power transmission network.
2. Description of Related Art
As processor capabilities have expanded and power requirements have decreased, there has been an ongoing explosion of devices that operate completely independent of wires or power cords. These “untethered” devices range from cell phones and wireless keyboards to building sensors and active Radio Frequency Identification (RFID) tags.
Engineers and designers of these untethered devices continue to have to deal with the limitations of portable power sources, primarily using batteries as the key design parameter. While the performance of processors and portable devices has been doubling every 18-24 months (driven by Moore's law), battery technology in terms of capacity has only been growing at 6% per year.
Even with power conscious designs and the latest in battery technology, many devices do not meet the lifetime cost and maintenance requirements for applications that require a large number of untethered devices, such as logistics and building automation. Today's devices that need two-way communication require scheduled maintenance every three to 18 months to replace or recharge the device's power source (typically a battery). One-way devices that simply broadcast their status without receiving any signals, such as automated utility meter readers, have a better battery life typically requiring replacement within 10 years. For both device types, scheduled power-source maintenance is costly and can be disruptive to the entire system that a device is intended to monitor and/or control. Unscheduled maintenance trips are even more costly and disruptive. On a macro level, the relatively high cost associated with the internal battery also reduces the practical, or economically viable, number of devices that can be deployed.
The ideal solution to the power problem for untethered devices is a device or system that can collect and harness sufficient energy from the environment. The harnessed energy would then either directly power an untethered device or augment a power supply. However, this ideal solution may not always be practical to implement due to low energy in the environment and site restrictions that limit the ability to use a dedicated energy supply.
A need exists for a system that takes these factors into account and provides a solution for both the ideal situation and also for more restrictive circumstances.
Previous inventions have focused on a parallel network for power distribution, for example, U.S. Provisional Patent Application Nos. 60/683,991 and 60/763,582, both entitled Power Transmission Network and incorporated by reference herein. These inventions did not explore a network in series because, for many applications exploiting this technology, losses from transmission lines, series switches, directional couplers (DC), and connectors are unacceptable. However, in certain applications, these losses are acceptable or may be minimized, for example, a small network with a coaxial cable infrastructure, such as a desk area, or using a new or existing low-loss coaxial cable infrastructure in a building for distributing RF power.